Thanks for your tricky question. Firstly, I applaud you for trying different strategies other than a small smack as that definitely does not work! If a child is in significant distress, this requires "time-in". Young children have not yet had the opportunity to develop the necessary skills to regulate their emotions and often need the help of an adult to calm down to learn how to regulate their emotions.
You say that the behaviour is about a 'cry for attention'. If this is the case the message behind the behaviour might be he needs to 'be seen more'. Some children need to be seen more than others. One effective strategy is to ‘name what he is doing’ when in play e.g. I can see you are playing with the ball (then wait). This positive attention (or naming what he is doing) gives the child the messages "I am seen; I have good play ideas and others are interested in me". Giving children individual attention or naming what they are doing when they are doing the "right thing" is often just as effective to set up a positive expectation of the behaviour that you want.
When you need to give your attention to your other children, you can "name what you are doing on a concrete level". For example, "I have been watching you play with the ball and now I am going to change your sister's nappy. You can join me and watch or you can stay here and continue to play with the ball". This is called "naming your initiaitives" and makes you more predictable to him and also gives him 'detailed guidance' of what he can do whilst you attend to his sister's needs.
Talking in this way feels uncomfortable at first but it sets everyone up for success. Over time you should see a reduction in "attention seeking" behaviours. It also supports language development of your children by increasing the number of words heard everyday in the home.
The parenting strategies of naming initiaitives, naming emotions, naming what you are doing; giving positive attention (time-in) should reduce the need for using time-out and build stronger relationships with your children.